1. Field of the Invention
The present invention relates to an apparatus and a method for preparing a thin film electrode of hydrous ruthenium oxide. More specifically, the present invention relates to an apparatus and a method for preparing an electrode of hydrous ruthenium oxide thin film by spraying ruthenium precursor solution on a heated substrate under strong electric field, thus forming a porous thin film electrode of very fine size.
2. Description of the Related Art
Generally, a high performance portable power supply has been used as a main component of end-products essentially used in all portable information communication equipment, electronic apparatus, electric automobile and so on. Next generation energy storage systems, which have been recently developed, utilize electrochemical principles and are exemplified by a Li based secondary cell and an electrochemical capacitor.
In the aspect of energy amount, the secondary cell is advantageous because it is capable of accumulating dense energy per unit weight or unit volume. However, the secondary cells are disadvantageous in light of energy amount (power density), applicable per time, period of use, and charging time. Furthermore, the electrochemical capacitor has energy density smaller than the secondary cell, and is more advantageous in terms of duration of use, charging time and power density, compared to the secondary cell. Therefore, in the case of electrochemical capacitor, research for improving energy density has been performed with vigor. The electrochemical capacitor with ultrahigh capacitance applied the electrochemical principle is classified into an electrical double layer capacitor (EDLC) using a principle of electrical double layer, and a super capacitor showing ultrahigh capacitance which has maximal capacitance 10 fold larger than that of EDLC by pseudocapacitance generated from electrochemical faradic reaction. In the case of the EDLC, activated carbon/fiber is useful as electrode material of the capacitor, and thus an electric charge of high density is accumulated in the electrical double layer. As the electrode material contained in the super capacitor, the metal oxides can be used.
A super capacitor with high capacitance and high power density can either be used alone or in combination with a secondary cell in the fields of power supplies for electro mobile, portable mobile communication equipment, memory back-up of computer, military and aeronautic apparatus, and micro medical equipment. EDLC that utilizes synthetic carbon material as electrode has been commercially available since the early 1980s in Japan, thus their practical developments have reached their technical limits. Regular research for metal oxide based electrode material utilizing super capacitor has only been embarked on within these past 4 to 5 years by both the United States and Japan. However, the super capacitors have some obvious setbacks, the setbacks are manifested in the form of expensive electrode active materials and difficult preparation process. Therefore, they can only be used for special purposes, such as military affairs. Its commercial use has been delayed.
On the other hand, the electrode materials developed for super capacitor until now comprise RuO2, IrO2, NiO, CoOx, MnO2, WO3 and the like. Of these materials, RuO2 and IrO2 show the most excellent electrochemical properties, while the other materials show poor electrochemical properties. Most researches have placed emphasis on these two materials. In particular, ruthenium oxide with very excellent performance has been widely studied as metal oxide electrode for super capacitor applications.
In general, ruthenium oxide is mainly divided into two types of the anhydrous type and the hydrous type.
To produce an electrode of ruthenium oxide for super capacitor, an anhydrous ruthenium oxide electrode is mainly prepared by pyrolysis process and a hydrous ruthenium oxide electrode is mainly prepared by sol-gel process. During the pyrolysis process, RuCl3.xH2O dissolved in distilled water or alcohol is used as a precursor solution, which is then pyrolyzed on a conductive metal substrate at high temperature 350–500° C. As a result of the process, tantalum or titanium is used as substrate material, particularly; titanium is preferred because of the excellent adhesion between titanium and RuO2. For the sol-gel process, a precursor solution is RuCl3.xH2O in distilled water, similar to the pyrolysis process aqueous NaOH solution is slowly added to maintain the solution at about 7. Ru(OH)3 is thermodynamically stabilized and precipitated in the solution in black powder form. The precipitated ruthenium powder is filtered with about 8 μm-caliber filter and then washed. The washing is performed by repeating the procedure five times, that is, the procedure of distilled water poured in to a powder contained vessel at a suitable volume, stirred for approxiamately thirty minutes and then filtered. The final product obtained is RuO2.2H2O, which is then thermally treated at appropriate temperature for 17 hours, thereby yielding hydrous ruthenium oxide powder. In order to employ the powder as a practical electrode, the powder is mixed with Teflon binder in a mass ratio of 5% and rolled, to prepare an electrode in a thin film form of 100–200 μm. Although the electrode of hydrous ruthenium oxide has high capacitance of 720 F/g, it has a disadvantage of poor discharge/charge and low power properties. To alleviate these disadvantages, powders prepared by sol-gel process are mechanically mixed with approximately 20% activated carbon and Teflon binder, to yield a composite electrode, which has specific capacitance of 650 F/g lower than that of hydrous ruthenium oxide electrode, at slow discharge rate, but maintaining a specific capacitance of about 600 F/g at fast discharge rate. However, sol-gel process has disadvantages because the process is complicated with multiple steps and long periods of time required for preparation.